The invention relates to cellular phones used in mobile communications systems. More particularly, the invention relates to a circuit and a method for controlling electrical characteristics of a signal transmitted by a dual band cellular phone.
Usually, cellular phones are designed to operate within the environment of one of several mobile communications networks. One example of such an environment is a mobile communications network according to a standard known as GSM (Global System for Mobile communications) created by the European Telecommunications Standards Institute (ETSI). In GSM, there is an assigned frequency band around 900 MHz for Standard GSM, and an assigned frequency band around 1800 MHz for DCS1800 (Digital Communications System, DCS) which can be considered to be a further mobile communications environment. Other environments include systems known as Advanced Mobile Phone System (AMPS) operating in a frequency band around 800 MHz, and as Personal Communications System (PCS) operating in a frequency band around 1900 MHz.
As the number of users of cellular phones increases, some operators have added capacity to their networks by including more than only one frequency band in their networks. In the case of the GSM system, for example, the frequency bands around 900 MHz and 1800 MHz are now used to expand the capacities of certain networks.
Manufacturer of cellular phones, therefore, are developing a new generation of cellular phones which are operable at, for example, two frequency bands. A cellular phone operable at two frequency bands is referred to as a dual band cellular phone. As the dual band cellular phone has to transmit signals at both frequency bands, duplication of functions is required. Duplication of functions, however, leads to higher manufacturing costs and ultimately to higher prices for the dual band cellular phones.